This invention relates to improved para positional selectivity in the catalyzed conversion of monosubstituted benzenes containing meta-directing substituents to disubstituted benzenes and, more particularly, to a process for improving para selectivity in the conversion of a monosubstituted benzene such as nitrobenzene to a disubstituted benzene such as dinitrobenzene comprising contacting under conversion conditions, for example, the nitrobenzene in the gas phase with an inorganic or organic substitution agent such as nitrogen dioxide in the presence of a catalyst composition comprising a crystalline molecular sieve material.
In U.S. Pat. No. 4,415,744 to Monsanto, aromatics including unsubstituted, monosubstituted, and disubstituted compounds are described which are catalytically nitrated in the vapor phase with nitrogen dioxide over a sulfur trioxide-treated alumina-silica-metal oxide combination, (Al.sub.2 O.sub.3).sub.a (SiO.sub.2).sub.b (M.sub.2 /O).sub.c. Combinations include aluminosilicates such as synthetic and naturally-occurring zeolites. An essential step in making this nitration promoting catalyst is its activation with sulfur trioxide. European Patent Application No. 0092372 to Sumitomo teaches the gas-phase nitration of benzene by nitrogen dioxide over an acidic mixed oxide containing not less than two metallic oxides and containing at least one component selected from the group WO.sub.3, MoO.sub.3 and TiO.sub.2. The catalysts are said to avoid production of by-products such as dinitrobenzene. Another European Patent Application, No. 0053031 to Monsanto, teaches the vapor-phase nitration of aromatic compounds "susceptible of existing in the vapor phase at temperatures less than 190.degree. C." in the presence of molecular sieve catalysts. Reaction temperature is limited to between 80.degree. and 190.degree. C. and ferrierite, Zeolite X, and Zeolite Y are named as useful catalysts. The use of a phosphorus-vanadium-oxygen complex to nitrate aromatics in the gas phase using nitrogen dioxide is taught by Monsanto in U.S. Pat. No. 4,347,389. In European Patent Application No. 0093522, acidic solid surfaces including those of zeolites are used to catalyze the vapor-phase nitration of organic compounds, primarily aliphatics, with a combination of nitrogen dioxide and hydrogen peroxide. The nitration of benzene or toluene using gasified nitric acid over acid catalysts of the zeolite type, preferably montmorillonite, is taught in British Pat. No. 2000141. This patent describes enhanced para selectivity in the nitration of toluene, which is ortho-para not meta directing, compared to a solution nitration process using a mixture of nitric and sulfuric acids. European Patent Application No. 0017560 describes the gas-phase nitration of lower-than-C.sub.5 paraffins using nitrogen peroxide, nitric acid, or compounds containing a transferable nitro or nitrosyl group. The use of nitric acid as a gas-phase nitrating agent, this time for toluene, is described in U.S. Pat. No. 4,112,006. In the U.S. Pat. No. 4,112,006 the process is carried out in the presence of a carrier substance based on silica and/or alumina which may additionally contain a minor quantity of a different inorganic oxide such as magnesia, the carrier being impregnated by a high boiling inorganic acid such as phosphoric acid or sulfuric acid and, optionally, a salt of such an acid, e.g., iron or aluminum sulfate or phosphate. U.S. Pat. No. 4,107,220 teaches controlling the ortho-para isomer distribution in the products during the catalyzed gas-phase nitration of chlorobenzene with an oxide of nitrogen such as nitrogen dioxide. The molecular sieve catalysts taught by the U.S. Pat. No. 4,107,220 are crystalline synthetic zeolites having a pore size from about 5 to about 10 .ANG. and include Zeolon 300 and 900, AW-500 sieve, 13.times. molecular sieve, etc. Finally, McKee and Wilhelm in Industrial and Engineering Chemistry 28, 6, 662-7 (1936) teach the catalyzed vapor-phase nitration of benzene and toluene over silica gel using a nitrogen oxide.
Zeolitic materials, both natural and synthetic, are known to have catalytic capabilities for many hydrocarbon conversion processes. Such materials typically are ordered porous crystalline aluminosilicates having a definite structure with cavities interconnected by channels. The cavities and channels throughout the crystalline material generally are uniform in size allowing selective separation of hydrocarbons. Consequently, these materials, in many instances, are known in the art as "molecular sieves" and are used, in addition to selective adsorptive processes, for certain catalytic properties. The catalytic properties of these materials are affected, to some extent, by the size of the molecules which selectively penetrate the crystal structure, presumably to contact active catalytic sites within the ordered structure of these materials.
Generally, the term "molecular sieve" includes a wide variety of both natural and synthetic positive-ion-containing crystalline zeolite materials. They generally are characterized as crystalline aluminosilicates which comprise networks of SiO.sub.4 and AlO.sub.4 tetrahedra in which silicon and aluminum atoms are cross-linked by sharing of oxygen atoms. The negative framework charge resulting from substitution of an aluminum atom for a silicon atom is balanced by positive ions, for example, alkali-metal or alkaline-earth-metal cations, ammonium ions, or hydrogen ions.
Prior art developments have resulted in the formation of many synthetic zeolitic crystalline materials. Crystalline aluminosilicates are the most prevalent and, as described in the patent literature and in the published journals, are designated by letters or other convenient symbols. Examples of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-4 (U.S. Pat. No. 3,578,723), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), Zeolite ZSM-12 (U.S. Pat. No. 3,832,449), Zeolite NU-1 (U.S. Pat. No. 4,060,590), and others. In addition, although boron is not considered a replacement for aluminum or silicon in a zeolite composition, a borosilicate sieve is described in U.S. Pat. Nos. 4,268,420 and 4,269,813.
One of the problems in the reaction chemistry of monosubstituted aromatics is the directing effect of the substituent already present on the ring on the entering position of a second substituent. A substituent on a benzene ring can be classified according to its ability to direct a second substituent to one of the three different positions on the monosubstituted ring. For example, alkyl and halo substituents direct the second group largely to the 2 and 4 positions forming ortho and para derivatives. Nitro, carboxylic acid, and sulfonic acid substituents on the other hand direct the second substituent largely to the 3 position forming meta derivatives. The directing influence can be quite strong; for example, in the conventional liquid-phase nitration of nitrobenzene a mixture of dinitrobenzenes is produced which contains about 93 percent of the meta isomer.
Now it has been found that by carrying out a substitution reaction on a monosubstituted benzene containing a meta-directing group in the gas phase over a crystalline molecular sieve the meta-directing influence of the substituent can be substantially reduced and a high proportion of the unfavored para isomer formed at good conversion levels.